1. Field of the Invention
The invention relates to a Plutella xylostella baculovirus which may be used for the biological control of insect pests.
2. Description of the Prior Art
Chemical pesticides and fungicides are the most commonly used control agents for forest and agricultural insect pests and fungal diseases. In excess of 350 billion pounds of these agents are used annually in the United States to control pests and diseases in forestry, agriculture, and residential areas. Unfortunately, broad spectrum insecticides and fungicides have adverse impacts not only on their target organisms but also on beneficial insects and fungi and, consequently, on the entire ecosystem. Pest insects may also acquire resistance to such chemicals so that new pest insect populations can develop that are resistant to these pesticides. Furthermore, chemical residues pose environmental hazards and possible health concerns.
Interest in biological insect and fungal control agents is growing as a consequence of concerns regarding chemical pesticide use. The biological control of insect pests presents an alternative means of pest control which can play a role in integrated pest management and reduce dependence on chemical pesticides. Generally, natural control agents have little adverse ecological impact due to their specificity for the target host. Long term environmental hazards and health concerns are not a factor with biological control agents because chemical residues are not present. However, biological control agents may suffer from several disadvantages in comparison to chemical pesticides, including cost of production, efficacy, and stability.
Viruses that cause natural epizootic diseases within insect populations are among the entomopathogens which have been developed as biological pesticides. A variety of viruses, including baculoviruses, are known to be valuable biological control agents for insects. Baculoviruses are a large group of viruses which are infectious only in arthropods (L. K. Miller, Virus Vector for Genetic Engineering in Invertebrates, in "Genetic Engineering in the Plant Sciences", N. Panopoulous, Ed., Praeger Publ., N.Y., pp. 203-224, 1981; Carstens, 1980, "Baculoviruses--Friend of Man, Foe of Insects?," Trends and Biochemical Science, 52:107-110; Harrap and Payne, "The Structural Properties and Identification of Insect Viruses" in Advances in Virus Research, M. A. Lawfer, F. B. Bang, K. Maramorosh and K. M. Smith, Eds., Vol. 25, pp. 273-355, Academic Press, New York, 1979).
In nature, infection is initiated when an insect ingests food contaminated with baculovirus particles, typically in the form of occlusion bodies (OB) which are composed of multiple viral particles embedded within a virus-encoded proteinaceous crystal. When consumed by susceptible insects, the protein crystal of the occlusion bodies is dissolved in the alkaline environment of the insect midgut, releasing individual virus particles which invade epithelial cells lining the midgut. Within the cell, the baculovirus migrates to the nucleus where replication takes place. Generally, two forms of baculovirus, occluded and extracellular virus (ECV), are produced during viral replication. Both of these viral particles as well as the OB itself possess envelopes. Initially, extracellular virus (ECV) is produced, acquiring an envelope as it buds out from the surface of the cell. This extracellular virus (ECV) can then infect other cells within the insect, including fat body cells, epidermal cells, and hemolymph. Following this initial stage of infection, virus is produced which is occluded in occlusion bodies. Occlusion body formation continues until the cell ultimately dies or lyses. Some baculoviruses infect virtually every tissue in the host insect so that at the end of the infection process, the entire insect is liquified, releasing extremely large numbers of occlusion bodies which are then responsible for spreading the infection to other insects. See "The Biology of Baculoviruses," R. G. Granados and B. A. Federici, Eds., Vol. I and II, CRC Press, Boca Raton, Fla., 1986.
Many baculoviruses infect insects which are pests of commercially important agricultural and forestry crops. Such baculoviruses are therefore potentially valuable as biological control agents. To date, several different baculoviruses have been registered for use as insecticides by the U.S. Environmental Protection Agency. One of these, Autographa californica multiple nuclear polyhedrosis virus (AcMNPV), is well recognized for its use as a biocontrol agent because of its wide host range within the order Lepidoptera. Another baculovirus, Anagrapha falcifera multiple nuclear polyhedrosis virus (AfMNPV), was recently isolated from a celery looper and differs from AcMNPV both in its REN pattern and its greater infectivity for Heliothis subflexa larvae. AfMNPV is approximately equally infectious for both H. zea and H. virescens larvae and possesses a wide range infecting over 30 species from 10 families in the order Lepidoptera.